7 Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya
thPeering into the Milky Way by FAST: Ionized gas in the Galactic disk revealedby the piggyback line observations of the FAST GPPS survey
L.G. Hou " , J.L. Han ",$ , Tao Hong " , X.Y. Gao ",$ and Chen Wang ""National Astronomical Observatories, Chinese Academy of Sciences,$School of Astronomy, University of Chinese Academy of Sciencese-mail (speaker): lghou@nao.cas.cn
Ionized gas, is one of the major components of theGalactic interstellar medium. It is widely distributed in
the Milky Way, accounting for about 20% of the total gas
mass ["] . However, the ionized gas in the Milky Way,
especially the low-density diffuse component, has not yet
been well revealed in the radio band, awaiting to beobserved with a much improve sensitivity, spatialresolution, spectral resolution, and sky coverage.
The Galactic Plane Pulsar Snapshot (GPPS) survey isan on-going pulsar hunting project made with theFive-hundred-meter Aperture Spherical radio Telescope(FAST) [$] , currently the largest single-dish telescope inthe world with a 300 m illumination aperture. The GPPS
survey has successfully discovered more than 600pulsars (see http://zmtt.bao.ac.cn/GPPS/ ) by using the 5
minutes integration of each pointing by using the L-band
19-beam receiver that has a system temperature of 22 K.
The GPPS survey is to make the most sensitive
systematic search for radio pulsars within the Galactic
latitude of ±10° around the Galactic plane visible by
FAST. During the pulsar survey observations, thepiggyback spectral line data in the L-band of 1000—
1500 MHz are recorded simultaneously with the digitalspectroscopy backends connected to the L-band 19-beamreceiver, providing the most sensitive spectra for morethan 150 radio recombination lines (RRLs) from theelements of Hydrogen (H), Helium (He), Carbon (C) andSulfur (S) etc.
The data processing pipeline for the Hnα RRLs havebeen developed by the research team. The firstpaper ['] publish the results in a pilot sky area of 88
square degrees in the inner Galaxy of 33° ≦ l ≦ 55°and |b| ≦ 2.0° (Figure 1). The spectral data of the HnαRRLs have a spatial resolution of about 3 arcminutes, aspectral resolution of 2.2 km s )" , and a typical spectral
RMS noise of 0.25 mJy beam)" or 6.3 mK in themain-beam brightness temperature. The piggyback HnαRRL observations of the FAST GPPS survey aresensitive down to an emission measure of about
200 cm)/ pc for the Galactic interstellar ionized gas atthe 3σ detection limit. From the first Hnα RRL map of
the observed sky area, we find complex structuralfeatures dominated by a number of HII regions as well as
extended ionized gas regions. We detect about 94% of
the known HII regions, and confirm 43 WISE candidates
as HII regions. Several large HII regions or star-forming
complexes in the distant outer Galaxy are resolved by theFAST observations of Hnα RRLs. Extended RRLfeatures of the low-density diffuse ionized gas aredetected. In addition, the piggyback spectral line data of
the GPPS project also provide sensitive observations forother kinds of RRLs at L-band, such as the Henα, Cnα,Snα, Hnβ and Hnγ RRLs, which will be graduallyprocessed and released on the project web-page:
http://zmtt.bao.ac.cn/MilkyWayFAST/.
As the most sensitive survey for RRLs at L-band todate, together with good spatial resolution, spectral
resolution and large sky coverage, the piggyback spectral
data of the FAST GPPS survey will promote ourunderstanding of the properties of ionized gas in theMilky Way.
References[1] J. Lequeux, “The interstellar medium”, ism..book.
(Springer, Berlin, 2005)[2] J.L. Han et al, Res. Astron. Astrophys. 21, 107 (2021)[3] L.G. Hou et al, Sci. China-Phys. Mech. Astron. 65,129703 (2022)

Figure 1. Velocity-integrated intensity map of the Hnα RRLs recorded by the FAST GPPS survey for the inner Galactic
plane of 33° ≦ l ≦ 55° and |b| ≦ 2.0°. The observation beam has a size of about 3 arcminutes. The velocity range
for the integration is from 0123 = −40 km s )" to 120 km s )" . The overlaid contour levels are at 2n × 5 mJy kms )" beam)" , with n = 2, 3, ..., 10.